Managing memory transactions in a distributed shared memory system supporting caching above a point of coherency

ABSTRACT

In response to execution in a memory transaction of a transactional load instruction that speculatively binds to a value held in a store-through upper level cache, a processor core sets a flag, transmits a transactional load operation to a store-in lower level cache that tracks a target cache line address of a target cache line containing the value, monitors, during a core TM tracking interval, the target cache line address for invalidation messages from the store-in lower level cache until the store-in lower level cache signals that the store-in lower level cache has assumed responsibility for tracking the target cache line address, and responsive to receipt during the core TM tracking interval of an invalidation message indicating presence of a conflicting snooped operation, resets the flag. At termination of the memory transaction, the processor core fails the memory transaction responsive to the flag being reset.

PRIORITY CLAIM

This application is a continuation of U.S. patent application Ser. No.14/278,005 entitled “MANAGING MEMORY TRANSACTIONS IN A DISTRIBUTEDSHARED MEMORY SYSTEM SUPPORTING CACHING ABOVE A POINT OF COHERENCY,”filed on May 15, 2014, the disclosure of which is incorporated herein byreference in its entirety for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates generally to data processing and, inparticular, to memory transactions in a distributed shared memory systemof a data processing system.

A conventional multiprocessor (MP) computer system, such as a servercomputer system, includes multiple processing units all coupled to asystem interconnect, which typically comprises one or more address, dataand control buses. Coupled to the system interconnect is a systemmemory, which represents the lowest level of volatile memory in themultiprocessor computer system and which generally is accessible forread and write access by all processing units. In order to reduce accesslatency to instructions and data residing in the system memory, eachprocessing unit is typically further supported by a respectivemulti-level cache hierarchy, the lower level(s) of which may be sharedby one or more processor cores.

Cache memories are commonly utilized to temporarily buffer memory blocksthat might be accessed by a processor in order to speed up processing byreducing access latency introduced by having to load needed data andinstructions from system memory. In some MP systems, the cache hierarchyincludes at least two levels. The level one (L1) or upper-level cache isusually a private cache associated with a particular processor core andcannot be accessed by other cores in an MP system. Typically, inresponse to a memory access instruction such as a load or storeinstruction, the processor core first accesses the directory of theupper-level cache. If the requested memory block is not found in theupper-level cache, the processor core then accesses lower-level caches(e.g., level two (L2) or level three (L3) caches) or system memory forthe requested memory block. The lowest level cache (e.g., L3 cache) isoften shared among several processor cores.

In such systems, multiprocessor software concurrently accesses shareddata structures from multiple software threads. When concurrentlyaccessing shared data it is typically necessary to prevent so-called“unconstrained races” or “conflicts”. A conflict occurs between twomemory accesses when they are to the same memory location and at leastone of them is a write and there is no means to ensure the ordering inwhich those accesses occur.

Multiprocessor software typically utilizes lock variables to coordinatethe concurrent reading and modifying of locations in memory in anorderly conflict-free fashion. A lock variable is a location in memorythat is read and then set to a certain value, possibly based on thevalue read, in an atomic fashion. The read-modify-write operation on alock variable is often accomplished utilizing anatomic-read-modify-write (ARMW) instruction or by a sequence ofinstructions that provide the same effect as a single instruction thatatomically reads and modifies the lock variable.

In this manner, a software thread reading an initial “unlocked” valuevia an ARMW instruction is said to have “acquired” the lock and will,until it releases the lock, be the only software thread that holds thelock. The thread holding the lock may safely update the shared memorylocations protected by the lock without conflict with other threadsbecause the other threads cannot obtain the lock until the currentthread releases the lock. When the shared locations have been readand/or modified appropriately, the thread holding the lock releases thelock (e.g., by writing the lock variable to the “unlocked” value) toallow other threads to access the shared locations in storage.

While locking coordinates competing threads' accesses to shared data,locking suffers from a number of well known shortcomings. These include,among others, (1) the possibility of deadlock when a given thread holdsmore than one lock and prevents the forward progress of other threadsand (2) the performance cost of lock acquisition when the lock may nothave been strictly necessary because no conflicting accesses would haveoccurred to the shared data.

To overcome these limitations, the notion of transactional memory can beemployed. In transactional memory, a set of load and/or storeinstructions are treated as a “memory transaction” (also referred toherein simply as a “transaction.”) A transaction succeeds when theconstituent load and store operations can occur atomically without aconflict with another thread. The transaction fails in the presence of aconflict with another thread and can then be re-attempted. If atransaction continues to fail, software may fall back to using lockingto ensure the orderly access of shared data.

To support transactional memory, the underlying hardware tracks thestorage locations involved in the transaction—the transactionfootprint—as the transaction executes for conflicts. If a conflictoccurs in the transaction footprint, the transaction is aborted andpossibly restarted. Use of transactional memory reduces the possibilityof deadlock due to a thread holding multiple locks because, in thetypical case, no locks are held (the transaction simply attempts to makeone or more storage accesses and restarts if a conflict occurs).Further, the processing overhead of acquiring a lock is generallyavoided.

BRIEF SUMMARY

According to one embodiment, a processing unit for a multiprocessor dataprocessing system includes a processor core including a store-throughupper level cache, an instruction execution unit, and a flag. Inresponse to execution in a memory transaction of a transactional loadinstruction that speculatively binds to a value held in thestore-through upper level cache, the processor core sets the flag,transmits a transactional load operation to a store-in lower level cachethat tracks a target cache line address of a target cache linecontaining the value, monitors, during a core TM tracking interval, thetarget cache line address for invalidation messages from the store-inlower level cache until the store-in lower level cache signals that thestore-in lower level cache has assumed responsibility for tracking thetarget cache line address, and responsive to receipt during the core TMtracking interval of an invalidation message indicating presence of aconflicting snooped operation, resets the flag. At termination of thememory transaction, the processor core fails the memory transactionresponsive to the flag being reset.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a high-level block diagram of an exemplary data processingsystem in accordance with one embodiment;

FIG. 2 is a more detailed block diagram of an exemplary processing unitin accordance with one embodiment;

FIG. 3 is a detailed block diagram of lower level cache memorysupporting memory transactions in accordance with one embodiment;

FIG. 4A is an illustrative example of a memory transaction in accordancewith one embodiment;

FIG. 4B is a high level logical flowchart of an exemplary method bywhich a multiprocessor data processing system executes an instructioninitiating a memory transaction in accordance with one embodiment;

FIG. 5A depicts execution of an exemplary program illustrating causalityin a multiprocessor data processing system;

FIG. 5B illustrates execution of an exemplary program including memorytransactions to ensure causality;

FIG. 5C depicts execution of an exemplary program including bothtransactional and non-transactional memory accesses;

FIG. 6 illustrates a multiprocessor data processing system including atleast three processor cores that execute the exemplary program of FIG.5C;

FIG. 7 is a high level logical flowchart of an exemplary method by whicha processor core of a multiprocessor data processing system processes atransactional load instruction in accordance with one embodiment;

FIG. 8 is a high level logical flowchart of an exemplary method by whicha lower level cache memory of a multiprocessor data processing systemprocesses a transactional load operation in accordance with oneembodiment;

FIG. 9 is a high level logical flowchart of an exemplary method by whicha multiprocessor data processing system commits or fails a memorytransaction in accordance with one embodiment; and

FIG. 10 is a data flow diagram illustrating a design process.

DETAILED DESCRIPTION

With reference now to the figures, wherein like reference numerals referto like and corresponding parts throughout, and in particular withreference to FIG. 1, there is illustrated a high level block diagramdepicting an exemplary data processing system 100 in accordance with oneembodiment. In the depicted embodiment, data processing system 100 is acache coherent symmetric multiprocessor (SMP) data processing systemincluding multiple processing nodes 102 a, 102 b for processing data andinstructions. Processing nodes 102 are coupled to a system interconnect110 for conveying address, data and control information. Systeminterconnect 110 may be implemented, for example, as a busedinterconnect, a switched interconnect or a hybrid interconnect.

In the depicted embodiment, each processing node 102 is realized as amulti-chip module (MCM) containing four processing units 104 a-104 d,each preferably realized as a respective integrated circuit. Theprocessing units 104 within each processing node 102 are coupled forcommunication to each other and system interconnect 110 by a localinterconnect 114, which, like system interconnect 110, may beimplemented, for example, with one or more buses and/or switches. Systeminterconnect 110 and local interconnects 114 together form a systemfabric.

As described below in greater detail with reference to FIG. 2,processing units 104 each include a memory controller 106 coupled tolocal interconnect 114 to provide an interface to a respective systemmemory 108. Data and instructions residing in system memories 108 cangenerally be accessed, cached and modified by a processor core in anyprocessing unit 104 of any processing node 102 within data processingsystem 100. System memories 108 thus form the lowest level of volatilestorage in the distributed shared memory system of data processingsystem 100. In alternative embodiments, one or more memory controllers106 (and system memories 108) can be coupled to system interconnect 110rather than a local interconnect 114.

Those skilled in the art will appreciate that SMP data processing system100 of FIG. 1 can include many additional non-illustrated components,such as interconnect bridges, non-volatile storage, ports for connectionto networks or attached devices, etc. Because such additional componentsare not necessary for an understanding of the described embodiments,they are not illustrated in FIG. 1 or discussed further herein. Itshould also be understood, however, that the enhancements describedherein are applicable to cache coherent data processing systems ofdiverse architectures and are in no way limited to the generalized dataprocessing system architecture illustrated in FIG. 1.

Multiprocessor data processing systems such as data processing system100 of FIG. 1 implement a memory consistency model that specifies thelegal possible executions of a given multiprocessor program with respectto memory accesses (e.g., among other things, the values that may bereturned by load instructions, the order of writes to memory, thoseinstruction execution dependencies that affect the ordering of memoryaccesses, and the final values for memory locations at the conclusion ofa multiprocessor program). A memory consistency model is specified bytwo major characteristics: (1) ordering of memory access operations and(2) atomicity of store operations.

The ordering of memory operations specifies how memory operations may,if at all, be re-ordered relative to the order of their respective loadand store instructions in the individual threads of execution in themultiprocessor program. Memory consistency models must define orderingof memory access operations in four general cases: (1) ordering of thememory operations for a load instruction to a following loadinstruction, (2) ordering of the memory operations for a loadinstruction to a following store instruction, (3) ordering of the memoryoperations for a store instruction to a following store instruction, and(4) ordering of the memory operations for a store instruction to afollowing load instruction. Strong consistency memory models will, ingeneral, preserve all or at least most of these orderings. Inparticular, many strong consistency memory models enforce the firstthree orderings, but do not enforce store-to-load ordering. Weakconsistency memory models will generally not enforce most or all ofthese orderings.

Atomicity of store operations refers to whether or not a given thread ofexecution can read the value of its own store operation before otherthreads, and furthermore, whether the value written to the distributedshared memory system by the store operation becomes visible to otherthreads in a logically instantaneous fashion or whether the value canbecome visible to other threads at different points in time. A memoryconsistency model is called “multi-copy atomic” if the value written bya store operation of one thread becomes visible to all other threads ina logically instantaneous fashion. In general, strong consistency memorymodels are multi-copy atomic, and weak consistency memory models do notenforce multi-copy atomicity.

In a given multiprocessor program, program semantics often require thatmulti-copy atomicity and/or the various orderings between memory accessoperations are respected. Therefore, in a data processing system 100having a distributed shared memory system that implements a weakconsistency memory model, so called “barrier” (e.g., SYNC) instructionsare typically provided to allow the programmer to specify what memoryaccess operation orderings and atomicity are to be applied duringexecution of the multiprocessor program.

Referring now to FIG. 2, there is depicted a more detailed block diagramof an exemplary processing unit 104 in accordance with one embodiment.In the depicted embodiment, each processing unit 104 is an integratedcircuit including two or more processor cores 200 a, 200 b forprocessing instructions and data. In a preferred embodiment, eachprocessor core 200 is capable of independently executing multiplehardware threads of execution simultaneously. However, in the followingdescription, unless the interaction between threads executing on a sameprocessor core is relevant in a particular context, for simplicity,terms “processor core” and “thread executing on a processor core” areused interchangeably. As depicted, each processor core 200 includes oneor more execution units, such as load-store unit (LSU) 202, forexecuting instructions. The instructions executed by LSU 202 includememory access instructions that request load or store access to a memoryblock in the distributed shared memory system or cause the generation ofa request for load or store access to a memory block in the distributedshared memory system. Memory blocks obtained from the distributed sharedmemory system by load accesses are buffered in one or more registerfiles (RFs) 208, and memory blocks updated by store accesses are writtento the distributed shared memory system from the one or more registerfiles 208.

The operation of each processor core 200 is supported by a multi-levelvolatile memory hierarchy having at its lowest level a shared systemmemory 108 accessed via an integrated memory controller 106, and at itsupper levels, one or more levels of cache memory, which in theillustrative embodiment include a store-through level one (L1) cache 226within and private to each processor core 200, and a respective store-inlevel two (L2) cache 230 for each processor core 200 a, 200 b. In orderto efficiently handle multiple concurrent memory access requests tocacheable addresses, each L2 cache 230 can be implemented with multipleL2 cache slices, each of which handles memory access requests for arespective set of real memory addresses.

Although the illustrated cache hierarchies includes only two levels ofcache, those skilled in the art will appreciate that alternativeembodiments may include additional levels (L3, L4, etc.) of on-chip oroff-chip, private or shared, in-line or lookaside cache, which may befully inclusive, partially inclusive, or non-inclusive of the contentsthe upper levels of cache.

Each processing unit 104 further includes an integrated and distributedfabric controller 216 responsible for controlling the flow of operationson the system fabric comprising local interconnect 114 and systeminterconnect 110 and for implementing the coherency communicationrequired to implement the selected cache coherency protocol. Processingunit 104 further includes an integrated I/O (input/output) controller214 supporting the attachment of one or more I/O devices (not depicted).

In operation, when a hardware thread under execution by a processor core200 includes a memory access instruction requesting a specified memoryaccess operation to be performed, LSU 202 executes the memory accessinstruction to determine the target address (e.g., an effective address)of the memory access operation. After translation of the target addressto a real address, L1 cache 226 is accessed utilizing the targetaddress. Assuming the indicated memory access cannot be satisfied solelyby reference to L1 cache 226, LSU 202 then transmits the memory accessrequest, which includes at least a transaction type (ttype) (e.g., loador store) and the target real address, to its affiliated L2 cache 230for servicing.

With reference now to FIG. 3, there is illustrated a more detailed blockdiagram of an exemplary embodiment of a lower level cache (e.g., an L2cache 230) that supports memory transactions in accordance with oneembodiment. As shown in FIG. 3, L2 cache 230 includes a cache array 302and a directory 308 of the contents of cache array 302. Although notexplicitly illustrated, cache array 302 preferably is implemented with asingle read port and single write port to reduce the die area requiredto implement cache array 302.

Assuming cache array 302 and directory 308 are set associative as isconventional, memory locations in system memories 108 are mapped toparticular congruence classes within cache array 302 utilizingpredetermined index bits within the system memory (real) addresses. Theparticular memory blocks stored within the cache lines of cache array302 are recorded in cache directory 308, which contains one directoryentry for each cache line. While not expressly depicted in FIG. 3, itwill be understood by those skilled in the art that each directory entryin cache directory 308 includes various fields, for example, a tag fieldthat identifies the real address of the memory block held in thecorresponding cache line of cache array 302, a state field thatindicates the coherency state of the cache line, an LRU (Least RecentlyUsed) field indicating a replacement order for the cache line withrespect to other cache lines in the same congruence class, andinclusivity bits indicating whether the memory block is held in theassociated L1 cache 226.

L2 cache 230 includes multiple (e.g., 16) Read-Claim (RC) machines 312for independently and concurrently servicing load (LD) and store (ST)requests received from the affiliated processor core 200. In order toservice remote memory access requests originating from processor cores200 other than the affiliated processor core 200, L2 cache 230 alsoincludes multiple snoop machines 311. Each snoop machine 311 canindependently and concurrently handle a remote memory access request“snooped” from local interconnect 114. As will be appreciated, theservicing of memory access requests by RC machines 312 may require thereplacement or invalidation of memory blocks within cache array 302 (andL1 cache 226). Accordingly, L2 cache 230 also includes CO (castout)machines 310 that manage the removal and writeback of memory blocks fromcache array 302.

L2 cache 230 further includes an arbiter 305 that controls multiplexersM1-M2 to order the processing of local memory access requests and memorytransaction requests (corresponding to the tbegin and tend instructionsdescribed further herein) received from the affiliated processor core200 and remote requests snooped on local interconnect 114. Suchrequests, including local load and store and memory transaction requestsand remote load and store requests, are forwarded in accordance with thearbitration policy implemented by arbiter 305 to dispatch logic, such asa dispatch pipeline 306, which processes each read/load and storerequest with respect to directory 308 and cache array 302. As describedfurther below, transactional memory (TM) logic 380 processes memorytransaction requests and tracks memory access operations within memorytransactions to ensure completion of the memory access operations in anatomic manner or to abort the memory transactions in the presence ofconflicts.

L2 cache 230 also includes an RC queue 320 and a CPI (castout pushintervention) queue 318 that respectively buffer data being insertedinto and removed from the cache array 302. RC queue 320 includes anumber of buffer entries that each individually correspond to aparticular one of RC machines 312 such that each RC machine 312 that isdispatched retrieves data from only the designated buffer entry.Similarly, CPI queue 318 includes a number of buffer entries that eachindividually correspond to a particular one of the castout machines 310and snoop machines 311, such that each CO machine 310 and each snooper311 that is dispatched retrieves data from only the respectivedesignated CPI buffer entry.

Each RC machine 312 also has assigned to it a respective one of multipleRC data (RCDAT) buffers 322 for buffering a memory block read from cachearray 302 and/or received from local interconnect 114 via reload bus323. The RCDAT buffer 322 assigned to each RC machine 312 is preferablyconstructed with connections and functionality corresponding to thememory access requests that may be serviced by the associated RC machine312. RCDAT buffers 322 have an associated store data multiplexer M4 thatselects data bytes from among its inputs for buffering in the RCDATbuffer 322 in response unillustrated select signals generated by arbiter305.

In operation, a processor core 200 transmits store requests comprising atransaction type (ttype), target real address and store data to a storequeue (STQ) 304. From STQ 304, the store data are transmitted to storedata multiplexer M4 via data path 324, and the transaction type andtarget address are passed to multiplexer M1. Multiplexer M1 alsoreceives as inputs processor load requests from processor core 200 anddirectory write requests from RC machines 312. In response tounillustrated select signals generated by arbiter 305, multiplexer M1selects one of its input requests to forward to multiplexer M2, whichadditionally receives as an input a remote request received from localinterconnect 114 via remote request path 326. Arbiter 305 scheduleslocal and remote memory access requests for processing and, based uponthe scheduling, generates a sequence of select signals 328. In responseto select signals 328 generated by arbiter 305, multiplexer M2 selectseither the local request received from multiplexer M1 or the remoterequest snooped from local interconnect 114 as the next memory accessrequest to be processed.

The request selected for processing by arbiter 305 is placed bymultiplexer M2 into dispatch pipeline 306. Dispatch pipeline 306preferably is implemented as a fixed duration pipeline in which each ofmultiple possible overlapping requests is processed for a predeterminednumber of clock cycles (e.g., 4 cycles). During the first cycle ofprocessing within dispatch pipeline 306, a directory read is performedutilizing the request address to determine if the request address hitsor misses in directory 308, and if the memory address hits, thecoherency state of the target memory block. The directory information,which includes a hit/miss indication and the coherency state of thememory block, is returned by directory 308 to dispatch pipeline 306 in asubsequent cycle. As will be appreciated, no action is generally takenwithin an L2 cache 230 in response to miss on a remote memory accessrequest; such remote memory requests are accordingly discarded fromdispatch pipeline 306. However, in the event of a hit or miss on a localmemory access request or a hit on a remote memory access request, L2cache 230 will service the memory access request, which for requeststhat cannot be serviced entirely within processing unit 104, may entailcommunication on local interconnect 114 via fabric controller 216.

At a predetermined time during processing of the memory access requestwithin dispatch pipeline 306, arbiter 305 transmits the request addressto cache array 302 via address and control path 330 to initiate a cacheread of the memory block specified by the request address. The memoryblock read from cache array 302 is transmitted via data path 342 toError Correcting Code (ECC) logic 344, which checks the memory block forerrors and, if possible, corrects any detected errors. For processorload requests, the memory block is also transmitted to load datamultiplexer M3 via data path 340 for forwarding to the affiliatedprocessor core 200.

At the last cycle of the processing of a memory access request withindispatch pipeline 306, dispatch pipeline 306 makes a dispatchdetermination based upon a number of criteria, including (1) thepresence of an address collision between the request address and aprevious request address currently being processed by a castout machine310, snoop machine 311 or RC machine 312, (2) the directory information,and (3) availability of an RC machine 312 or snoop machine 311 toprocess the memory access request. If dispatch pipeline 306 makes adispatch determination that the memory access request is to bedispatched, the memory access request is dispatched from dispatchpipeline 306 to an RC machine 312 or a snoop machine 311. If the memoryaccess request fails dispatch, the failure is signaled to the requestor(e.g., local or remote processor core 200) by a retry response. Therequestor may subsequently retry the failed memory access request, ifnecessary.

While an RC machine 312 is processing a local memory access request, theRC machine 312 has a busy status and is not available to service anotherrequest. While an RC machine 312 has a busy status, the RC machine 312may perform a directory write to update the relevant entry of directory308, if necessary. In addition, the RC machine 312 may perform a cachewrite to update the relevant cache line of cache array 302. Directorywrites and cache writes may be scheduled by arbiter 305 during anyinterval in which dispatch pipeline 306 is not already processing otherrequests according to the fixed scheduling of directory reads and cachereads. When all operations for the given request have been completed,the RC machine 312 returns to an unbusy state.

Associated with RC machines 312 is data handling circuitry, differentportions of which are employed during the servicing of various types oflocal memory access requests. For example, for a local load request thathits in directory 308, an uncorrected copy of the target memory block isforwarded from cache array 302 to the affiliated processor core 200 viadata path 340 and load data multiplexer M3 and additionally forwarded toECC logic 344 via data path 342. In the case of an ECC error in thetarget memory block obtained by the local load request, corrected datais forwarded to RCDAT buffer 322 via data path 346 and store datamultiplexer M4 and then from RCDAT 322 to affiliated processor core 200via data path 360 and load data multiplexer M3. For a local storerequest, store data is received within RCDAT buffer 322 from STQ 304 viadata path 324 and store data multiplexer M4, the store is merged withthe memory block read into RCDAT buffer 322 from cache array 302 via ECClogic 344 and store data multiplexer M4, and the merged store data isthen written from RCDAT buffer 322 into cache array 302 via data path362. In response to a local load miss or local store miss, the targetmemory block acquired through issuing a memory access operation on localinterconnect 114 is loaded into cache array 302 via reload bus 323,store data multiplexer M4, RCDAT buffer 322 (with store merge for astore miss) and data path 362.

Referring now to FIG. 4A, an illustrative example of a memorytransaction is depicted. Those skilled in the art will recognize thatthe particular semantics and instructions utilized to implement thevarious memory transactions described herein are but some of thenumerous possible implementations and that the disclosed techniques ofimplementing transactional memory are not dependent on the specificinstructions and instruction semantics employed.

Illustrative memory transaction 400 includes a tbegin instruction 402that initiates transactional processing, a transaction body 406including one or more memory access instructions that are to beperformed atomically (and optionally one or more additionalunillustrated instructions), and a tend instruction 408 that terminatestransactional processing. Memory transaction 400 additionally includes abranch instruction 404 immediately following tbegin instruction 402.When memory transaction 400 first executes, the condition code registerin processor core 200 upon which branch instruction 404 depends isinitialized to a value that causes the program branch indicated bybranch instruction 404 not to be taken and the flow of execution tocontinue to transaction body 406. As discussed below, in response tofailure of memory transaction 400, as determined, for example, prior toor at the processing of the tend instruction 408, the condition coderegister is set to a different value, and branch instruction 404 causesexecution to branch to a fail handler routine.

FIG. 4B shows an exemplary process by a processor core 200 processes atbegin instruction 402 of a memory transaction 400 in accordance withone embodiment. The exemplary method begins at block 410 and thenproceeds to block 412, which illustrates the processor core 200 that isexecuting tbegin instruction 402 setting a TM flag 212 in processor core200 to indicate that the processor core 200 is currently executing amemory transaction 400 that has not yet failed (block 412). In addition,the processor core 200 takes a checkpoint 210 of the architectedregister state of processor core 200, which can be utilized to restorethe state of processor core 200 in the event memory transaction 400fails (block 414). Further, the processor core 200 invokes tracking ofload and store instructions within the transaction body 406 (referred toherein as “transactional load” and “transactional store” instructions)to ensure they complete in an atomic fashion or that memory transaction400 fails in the presence of a conflict (block 416). Processor core 200can invoke tracking of the transactional memory accesses of transactionbody 406, for example, by transmitting a corresponding TBEGIN request tothe affiliated L2 cache 230. Thereafter, the processing of the tbegininstruction 402 ends at block 418 of FIG. 4B.

In the exemplary embodiment of a lower level cache memory depicted inFIG. 3, tracking of transactional memory access (e.g., load and store)instructions within transaction body 406 to ensure that they complete inan atomic fashion (or that memory transaction 400 fails in the presenceof a conflict) is performed by TM logic 380 of L2 cache 230. Inparticular, TM tracking logic 381 within TM logic 380 includes a numberof entries that indicate which cache lines in cache array 302 of L2cache 230 are included in the transaction footprint. The transactionfootprint includes two portions: the load footprint corresponding tocache lines touched solely by loads within transaction body 406 (e.g.,the cache line at address A in exemplary memory transaction 400) and thestore footprint corresponding to cache lines touched solely by storeinstructions or by both load and store instructions in transaction body406 (e.g., the cache line at address B in exemplary memory transaction400).

As further shown in FIG. 3, TM logic 380 further includes transactionalcontrol logic 382, which controls the sequencing of a memory transactionand provides a pass/fail indication 384 and an optional TM killedindication 385 to the associated processor core 200. Pass/failindication 384 indicates to processor core 200 whether or not the memorytransaction successfully committed to the distributed shared memorysystem at the execution of the tend instruction 408 at the end of memorytransaction 400. TM killed indication 385 indicates to processor core200 whether or not a conflict has occurred during the transaction. Inresponse to transactional control logic 382 asserting TM killedindication 385, processor core 200 may, as a performance optimization,optionally abort and restart memory transaction 400 prior to reachingtend instruction 408.

In response to pass/fail indication 384 (or optionally TM killedindication 385) indicating that a conflict has occurred during executionof memory transaction 400, processor core 200 re-establishes itsarchitected register state from the checkpoint 210 taken at theexecution of tbegin instruction 402, invalidates the tentativelymodified cache lines in the store footprint, clears TM tracking logic381, sets the condition code register such that branch instruction 404will be taken, and transfers control to branch instruction 404. Inaddition, processor core 200 sets a transaction failure cause register(not shown) in processor core 200 to indicate the cause of the memorytransaction's failure. The fail handler routine invoked by branchinstruction 404 may choose to re-attempt memory transaction 400 or fallback to more conventional locking mechanisms, optionally based on thecontent of the transaction failure cause register.

During the execution of a memory transaction, the values stored to thedistributed shared memory system by transaction body 406 (i.e., those inthe store footprint of the memory transaction) are visible only to thethread of the processor core 200 executing the memory transaction.Threads running on the same or other processor cores 200 will not seethese values until and only if the memory transaction successfullycommits.

For a memory transaction to successfully commit, the transactional loadand store instructions in transaction body 406 must complete in anatomic fashion (i.e., there must be no conflicts for the cache lines inthe memory transaction's load and store footprints) and the effects ofthe store instructions in transaction body 406 must propagate to allprocessing units 104 in data processing system 100 and invalidate anycached copies of those cache lines held in other processing units 104.If both of these conditions hold when tend instruction 408 is executed,transactional control logic 382 indicates to processor core 200 viapass/fail indication 384 that memory transaction 400 passed and commitsall stores performed in transaction body 406 to L2 cache 230, thusmaking them visible to all other threads and processor cores 200 in thesystem simultaneously.

In the following discussion, a load or store instruction will be called“transactional” if that load or store instruction occurs within thetransaction body 406 of a memory transaction 400. Similarly, a load orstore will be called “non-transactional” if it occurs outside atransaction body 406. In one exemplary embodiment, a conflict policy ofdata processing system 100 defines a conflict with another processorcore's memory access to occur for a given memory transaction in any oneof several possible cases. In a first case, a conflict occurs if anon-transactional store from another processor core 200 hits a cacheline within either the given memory transaction's load or storefootprint. In a second case, a conflict occurs if a transactional storefrom another processor core 200 hits a cache line within the givenmemory transaction's load footprint. In a third case, a conflict occursif a non-transactional load hits a cache line within the given memorytransaction's store footprint. In a fourth case, a conflict occurs if atransactional load from another processor core 200 hits a cache linewithin the given memory transaction's store footprint. In addition, thegiven memory transaction has a conflict if one of its transactionalstores hits an address already extant in the store footprint of anotherprocessor core's memory transaction. This exemplary conflict policy isbut one of several possible embodiments of transactional memory. Theabove conflict policy biases in favor of transactional stores overtransactional loads, while allowing transactional and non-transactionalloads to freely intermingle.

Referring now to FIG. 5A, the execution of an exemplary programillustrating the property of causality in a multiprocessor dataprocessing system is shown. As used herein “causality,” which isdesirable property in multiprocessor programs, is defined as beingpreserved if, during execution of a multiprocessor program, a giventhread of execution cannot read the effects of a computation before thewrites that caused the computation can be read by the given thread.

In the simplified example given in FIG. 5A (as well as those discussedbelow with reference to FIGS. 5B-5C), a multiprocessor program isexecuted by three processor cores 200 of data processing system 100,labeled for ease of reference as processor core 0, processor core 1 andprocessor core 2. In FIG. 5A, processor core 0 executes a storeinstruction 500 that writes a value of 1 to address A in the distributedshared memory system. This update of address A propagates to processorcore 1, and load instruction 510 executed by processor core 1 thereforereturns a value of 1. Even though the memory update made by storeinstruction 500 has propagated to processor core 1, that memory updatemay not yet have propagated to processor core 2. If store instruction514 executes on processor 1 and the associated memory update propagatesto processor 2 before the memory update of store instruction 500propagates to processor 2, causality would be violated because the storeof the value of 1 to address B, which is an effect of the store toaddress A, would be visible to processor core 2 before the memory updateassociated with causal store instruction 500 was visible to processorcore 2.

To ensure causality in a weak consistency memory model, barrierinstruction 512 (e.g., a SYNC) ensures that store instruction 514 doesnot take effect or begin propagating its memory update to otherprocessor cores until load instruction 510 has bound to its value. Inaddition, barrier instruction 512 also ensures that the memory updateassociated with store instruction 500 propagates to processor 2 beforethe memory update associated with store instruction 514. Thus, causalityis preserved because the cause of the computation (i.e., the memoryupdate of store instruction 500) is visible to processor core 2 beforethe result of the computation (i.e., the memory update of store 514). Abarrier instruction 522 is also executed by processor core 2 to ensurethat processor core 2 executes load instructions 520 and 524 and bindstheir values in order, thus guaranteeing that processor core 2 properlyobserves the memory updates made by processor core 0 and processor core1.

With reference now to FIG. 5B, an exemplary embodiment of themultiprocessor program of FIG. 5A rendered in terms of memorytransactions is illustrated. In FIG. 5B, the branch instructions to thememory transaction fail handler are omitted for clarity.

As illustrated, processor core 0 executes a memory transaction 530including a tbegin instruction 532, tend instruction 536, and atransaction body including a store instruction 534 that stores a valueof 1 to address A. Upon the execution of tend instruction 536, memorytransaction 500 successfully commits and makes the update to address Avisible to all the other processor cores simultaneously. In particular,by the time load instruction 542 of the memory transaction 540 executingon processor core 1 can read the value of 1 from address A, loadinstruction 554 of the memory transaction 550 executing on processorcore 2 must also be able to read the value of 1 for address A. Memorytransaction 540 then reads the value of 1 for address A, stores a valueof 1 to address B and successfully commits. Finally, load instruction552 of memory transaction 550 reads a value of 1 for address B, andgiven that memory transaction 540 read a value of 1 for A, loadinstruction 554 must also read a value of 1 for address A.

In order to make the memory updates of store instructions in asuccessful transaction visible to all other processor coressimultaneously, before that memory transaction can commit all the cacheline invalidates necessitated by the memory transaction must havepropagated through the data processing system such that any otherprocessor cores' now stale copies of the updated cache lines have beenremoved (e.g., invalidated) and can no longer be read by the otherprocessor cores. Without this requirement, a processor core could stillread a stale value for an updated memory location after the memorytransaction that updated the memory location committed. A processorcore, therefore, needs to ensure that the memory updates associated withits own transactional stores are fully propagated through the dataprocessing system to invalidate any stale cached copies beforecommitting a successful memory transaction in order to maintain thesemantics of memory transactions. As a consequence of the propagation ofthe memory updates inherent in the semantics of memory transactions,causality is trivially preserved when only memory transactions areutilized to access memory locations in a distributed shared memorysystem. However, when transactional and non-transactional code interacton the same shared variables, causality is not directly preserved byensuring that the memory updates made by a memory transaction arevisible simultaneously to all other processor cores.

Referring now to FIG. 5C, an illustrative multiprocessor program isdepicted that includes a mixture of transactional and non-transactionalaccesses to a distributed shared memory system. In the exemplarymultiprocessor program, processor core 0 executes a non-transactionalstore instruction 560 that unconditionally writes a value of 1 toaddress A in the distributed shared memory system. This value propagatesto processor core 1 and is read by transactional load instruction 572within the memory transaction 570 executed by processor core 1.Processor core 1 then executes a store instruction 574 within memorytransaction 570 that updates the cache line associated with address Band completes invalidating any stale cached copies of the cache lineassociated with address B (so that no other processor core holds a copyof the now stale cache line) and successfully commits memory transaction570 upon execution of tend instruction 576. Processor core 2 thenexecutes load instructions 580 and 584 to read, in order, the cachelines associated with addresses B and A, respectively, based on theordering enforced by barrier instruction 582. If transaction 570 onlyensures that its own memory updates are fully propagated through thedistributed shared memory system before committing, the memory update ofstore instruction 560 may or may not have propagated to processor core2. Therefore, in at least some operating scenarios, processor core 2could read a value of 1 for the cache line associated with address B andthe, now stale, initial value of 0 for the cache line associated withaddress A, thus violating causality. The same result would be obtainedif processor core 2 utilized transactional loads to read from addressesA and B, as depicted for processor 2 in FIG. 5B.

To guarantee causality, memory transaction 570 must ensure not only thatits own transactional stores are propagated throughout the entiredistributed shared memory system, but also that any non-transactionalstore that is read by a transactional load within the transaction hasalso propagated throughout the distributed shared memory system. (Memoryupdates of transactional writes that are read by the memory transactionare guaranteed to have propagated throughout the distributed sharedmemory system because those memory updates could not be read bytransaction 570 before they were visible to the entire distributedshared memory system). To ensure that the memory updates ofnon-transactional stores read by memory transaction 570 are alsopropagated throughout the distributed shared memory system, theprocessing of the tend instruction 576 of memory transaction 570 mustnot allow commitment of memory transaction 570 until the memory updateof any non-transactional store read by memory transaction 570 ispropagated throughout the distributed shared memory system.

With reference now to FIG. 6, there is illustrated a partial view ofdata processing system 100 of FIG. 1, which executes the multiprocessorprogram of FIG. 5C. In the view given in FIG. 6, processor cores 200 a,200 b and 200 c respectively correspond to processor cores 0, 1 and 2 ofFIG. 5C. Further, an instance of causality resolution logic 379 isinstantiated for and coupled to each instance of snooper 311, forexample, as a component of the L2 cache 230 affiliated with eachprocessor core 200.

Initially, processor core 200 c holds a cached copy of the initial value(e.g., 0) of memory location A in its L1 cache 226 c. Processor 200 abegins execution of the multiprocessor program of FIG. 5C by executingstore instruction 560. In response to execution of store instruction560, processor core 200 a transmits a store request to its L2 cache 230a, which allocates an RC machine 312 to service the store request. RCmachine 312 broadcasts the store request onto local interconnect 114,and snoop machine 311 c of the L2 cache 230 c affiliated with processorcore 200 c registers the store request, including the processing unitthat sourced the store request (i.e., the processing unit includingprocessor core 200 a). At this point, the memory update of storeinstruction 560 has not propagated to processor core 200 c, but isinstead queued for later processing, advantageously allowing processorcore 200 a to continue executing further instructions before the memoryupdate of store instruction 560 is fully propagated.

Processor core 200 b then executes load instruction 572 and, finding nocopy of the target cache line associated with address A in its L1 cache226 b, transmits a read request to its L2 cache 230 b. In response tothe read request, L2 cache 230 b allocates RC machine 312 b to servicethe read request. In response to a miss of the read request in L2 cache230 b, RC machine 312 b issues a read request onto local interconnect114 to obtain the current value for address A. L2 cache 230 a respondsto the read request and provides the current value of address A toprocessor core 200 b by cache-to-cache intervention. At this point aso-called “causality passing read” has occurred, that is, loadinstruction 572 has read the value of a store instruction that has notfully propagated through the entire distributed shared memory system. Toaccount for this fact and to protect causality, causality resolutionlogic 379 c in L2 cache 230 c notes the successful read interventionbetween the vertical cache hierarchies of processor cores 200 a and 200b for an address that is currently being invalidated by snoop machine311 c. In this manner causality resolution logic 379 c directly tracksthe causal dependency that processor 200 b and its vertical cachehierarchy has on the memory update of store instruction 560 completingits propagation.

Processor 200 b executes store instruction 574, which specifies anupdate of the value of address B to 1. In response to execution of storeinstruction 574, RC machine 312 b issues a store request correspondingto store instruction 574 on local interconnect 114. In absence of anexisting cached copy of the target cache line, memory controller 106supplies the current value of address B from system memory 108 inresponse to the store request, and RC machine 312 b updates L2 cache 230b accordingly. At this point processor core 1 executes tend instruction576 to attempt to successfully commit transaction 570 and places acorresponding TEND request on local interconnect 114 to ensure that allprior memory updates by transactional stores in memory transaction 570have been propagated throughout the distributed shared memory system andthat any memory updates by non-transactional stores read by memorytransaction 570 have similarly propagated throughout the distributedshared memory system. In this case, the memory update of storeinstruction 574 has fully propagated throughout the distributed sharedmemory system because no other caches held a copy of the cache lineassociated with address B. However, had any such copy existed and hadthe memory update not been fully complete, a snoop machine 311 in thosecaches, which noted the initial processor core 200 issuing the store,would be active and would provide a retry response to the snooped TENDrequest from that processor core 200 (forcing the TEND request to bereissued) until the invalidation of the cached copy of the cache linecompletes.

In the case at hand, the TEND request is not from the processor core 200that initiated the store request, and therefore snoop machine 311 c willnot provide a retry response to the TEND request. However, causalityresolution logic 379 c has a causal dependency for processor 200 b andits vertical cache hierarchy and issues on local interconnect 114 aretry response to the TEND request because the TEND request was issuedfrom a processor core 200 that was the recipient of a causality passingread of the same address that snoop machine 311 c is processing. In thismanner, causality resolution logic 379 directly tracks which processorcores 200 have a causality dependency due to reading a memory update ofa non-transactional store that was not fully completed for the processorcore with which causality resolution logic 379 is associated.

It should be noted that, in general, causality resolution logic 379 mustmaintain a list capable of representing all the processors cores 200 inthe data processing system to provide causality in cases in which thecausality dependency chain passes through more than one processor core(e.g., a test where a first processor stores a location, a secondprocessor reads that location and then stores a first flag variable, athird processor loads the first flag variable and writes a second flagin a transaction, and then a final thread reads the second flag and thenthe initial location). In such an implementation, a TEND request issuedfrom any processor core with a causal dependency on the target addressbeing invalidated by the snoop machine 311 associated with the instanceof causality resolution logic 379 is retried. In a large SMP, however,such an embodiment can be prohibitive in cost and many implementationsof causality resolution logic 379 only precisely track causal dependencychains of a certain fixed depth (e.g., two or three processors) and inthe presence of longer dependency chains resort to pessimisticallyretrying all TEND requests until the cache line invalidationsnecessitated by the store instruction have completed processing.

To summarize, causality resolution logic is utilized to detect theoccurrence of causal dependency chains, to a depth determined by theembodiment, on a pending store that has not completed processingthroughout the entire distributed shared memory system. These causaldependencies are utilized to stall the completion of TEND requests fromthose processor cores with a causal dependency on the incomplete(pending) stores. In this manner, the memory transaction cannot complete(and therefore make its own stores visible), until the stores the memorytransaction has read (i.e., those in the causal dependency chain of thememory transaction) have first completed throughout the distributedshared memory system. Only after these stores in the memorytransaction's causal dependency chain (and the transactional stores ofthe memory transaction itself, though this is guaranteed by snooper 311instead of causality resolution logic 379) have completed, may the TENDrequest complete, leading to the memory transaction successfullycommitting if no conflicts have occurred during its execution.

In other embodiments, additional causality resolution logic may berequired to ensure the causality of memory operations. For example, inan implementation that contains a write-through L1 cache shared by amultithreaded processor core followed by a shared L2 store queue, it ispossible for different threads (i.e., logically different processorcores from the point of view of software) to read stored values from theL1 cache before these stores have even propagated to the L2 cache, muchless to the entire distributed shared memory system. In such animplementation, the tend instruction must act as a barrier fortransactional stores in the given thread. This behavior ensures that thetransactional stores are propagated to the system interconnect and thenecessary snoop machines 311 so that the tend instruction can ensure,when trying to complete the memory transaction, that all of the cacheline invalidations required by the memory transaction's stores havefully propagated. In addition, the tend instruction must act as abarrier for non-transactional stores that have been (or may have been)read by transactional loads within the transaction. In the simplest (andmost common embodiment), all non-transactional stores within the sharedstore queue are treated as if they have come from a single thread forpurposes of retrying the TEND request.

In this manner, all non-transactional stores from which any transactionhas (or may have) read that have not been fully propagated are broadcastto snoop machines 311 as necessary before a TEND request for anytransaction from that multithreaded processor core is presented on localinterconnect 114. In such an embodiment, snoop machines 311 treat allstores coming from a given multithreaded processor core in a unifiedmanner and will retry any TEND request, as necessary, from that givenmultithreaded processor core regardless of thread. In this embodiment,causality resolution logic 379 is not involved in monitoring theseintra-core dependencies, but instead is utilized solely to managecausality dependencies between multithreaded processor cores.

The exact placement and details of the necessary causality resolutionlogic will vary with the particulars of given embodiment and will beapparent to those skilled in the art given the teachings herein. Ingeneral, at any point where a load may return the value of a store thathas not fully propagated throughout the entire distributed shared memorysystem, a mechanism must be provided to ensure that any store with acausal dependency to a different processor core is noted and that causaldependency delays the processing of a tend instruction (or othersemantic) ending a memory transaction until such time as the stores inthe causal dependency chain of the memory transaction have completedpropagating.

As noted above, in the exemplary distributed shared memory systemdepicted in FIGS. 1-3, L1 caches 226 are implemented as store-throughcaches, meaning that memory coherence is managed at L2 caches 230.Consequently, decisions regarding whether memory transactions should becommitted to the distributed shared memory system or failed are alsomade at L2 caches 230. However, in certain operating scenarios, atransactional load request can hit in an L1 cache 226 and access therequested data before TM tracking logic 381 of the associated L2 cache230 is aware of the transactional load request. Further, prior toawareness of the transactional load request by L2 cache 230, the dataaccessed from L1 cache 226 by the transactional load request may berendered stale by receipt at the L2 cache 230 of a snooped operationthat invalidates the data. According to a preferred embodiment,responsibility for detecting such accesses to stale data in L1 caches226 is accordingly handled by processor cores 200 during a core TMtracking interval that begins when the transactional load hits in L2cache 226 and ends when the target cache line address of thetransactional load request is registered with the lower level cacheforming the point of coherency (e.g., L2 cache 230) and is thereafterhandled by the lower level cache (e.g., via TM tracking logic 381).

As described in greater detail below, in one embodiment, a processorcore 200 notifies the associated L2 cache 230 that a transactional loadrequest has accessed data from L1 cache 226 by issuing a specialinformational TM load request (referred to herein as a TM LDA request)that registers the target cache line address of the transactional loadrequest with L2 cache 230 as part of the transaction footprint, but doesnot request any return data. In the core TM tracking interval, processorcore 200 is responsible for determining whether or not the valueaccessed from L1 cache 226 has been corrupted by an invalidation messagesent by the associated L2 cache 230 and for recording any such datacorruption by resetting TM flag 212. In response to registration of thetarget cache line address of the transactional load request in L2 cache230 (e.g., in TM tracking logic 381), L2 cache 230 informs processorcore 200 that its responsibility for tracking (and the core TM trackinginterval) has ended by sending a TM LDA Ack message confirmingregistration of the target cache line address of the TM LDA request withL2 cache 230.

Once the target cache line address of a transactional load operation hasbeen registered with L2 cache 230 during a given memory transaction, itis unnecessary to again register the same target cache line address withL2 cache 230 during the same memory transaction. Because it is commonfor a memory transaction to include multiple transactional load requeststargeting the same cache line, in some implementations, a processor core200 could send multiple redundant TM LDA requests to register the sametarget cache line address with TM tracking logic 381 of L2 cache 230.However, in a preferred embodiment, each processor core 200 includes atransactional memory load directory (e.g., TM LD Dir 204) that filtersout the transmission of redundant TM LDA requests based on the targetcache line addresses of the last N transactional load requests that havehit in L1 cache 226. A replacement mechanism such as LRU 206 (or in asimpler embodiment a first-in, first-out (FIFO) queue) can be used tokeep a list of the cache lines most recently referenced by transactionalload operations fresh in TM LD Dir 204.

With reference now to FIG. 7, there is depicted a high level logicalflowchart of an exemplary method by which a processor core 200 of dataprocessing system 100 processes a transactional load instruction inaccordance with an embodiment of the present invention. As shown, theprocess begins at block 700 and thereafter proceeds to block 702, whichillustrates LSU 202 of processor core 200 receiving a transactional loadinstruction and then executing the transactional load instruction tocalculate a load target address.

Following execution of the transactional load instruction to determinethe load target address, processor core 200 accesses the directory of L1cache 226 using the load target address to determine whether a cacheline containing the data word(s) associated with the load target addressresides in (i.e., whether the load target address “hits”) L1 cache 226(block 704). If not, L1 cache 226 signals a cache miss, and processorcore 200 responds by issuing a transactional load (TM LD) operation toan RC machine 312 in L2 cache 230, as shown at block 706.

Following issuance of the TM LD operation to L2 cache 230, processorcore 200 receives the cache line of data identified by the load targetaddress of the TM LD operation from L2 cache 230. In response to receiptof the target cache line, processor core 200 installs the entire cacheline of data in L1 cache 226 and transfers the specific data word(s)requested by the transactional load instruction into core register(s)within register file 208 (block 708). Thus, unlike some prior artprocessors that only load the specifically requested data word(s) intocore register(s) and do not cache the entire target cache line,processor core 200 of data processing system 100 preferably establishesthe entire target cache line in L1 cache 226 to permit subsequent loads,and particularly subsequent transactional loads, to hit in L1 cache 226.Thereafter, the process terminates at block 710.

Returning to block 704, if processor core 200 determines that the loadtarget address hit in L1 cache 226, processor core 200 additionallydetermines at block 720 whether the target cache line address hits in TMLD Dir 204, indicating that a TM LDA operation has been sent to TMtracking logic 381 of L2 cache 230 in order to register the target cacheline address as part of the transaction footprint of the current memorytransaction. If so, processor core 200 need not again register thetarget cache line address with TM tracking logic 381. Accordingly, inresponse to an affirmative determination at block 720, the processpasses to block 722, which illustrates processor core 200 refrainingfrom transmitting a TM LDA operation to L2 cache 230 and instead simplyupdating LRU 206 of TM LD Dir 204 to reflect the access to the targetcache line. As shown at block 724, processor core 200 also installs thespecific data word(s) requested by the transactional load into one ormore core register(s) within register file 208. Thereafter, the processof FIG. 7 ends at block 710.

Returning to block 720, in response to a determination that the targetcache line address of the TM LD operation misses in TM LD Dir 204,processor core 200 issues an address-only TM LDA operation to L2 cache230 to inform TM tracking logic 381 of L2 cache 230 that thetransactional load operation has speculatively bound to a (potentiallystale) value in L1 cache 226 (block 730). Thus, the TM LDA operationrequests no return of data. As further illustrated at block 730,processor core 200 installs the target cache line address in TM LD Dir204 (performing any required eviction by reference to LRU 206) andupdates LRU 206 to reflect access to the target cache line. In addition,as shown at block 732, processor core 200 installs the specific dataword(s) requested by the transactional load into one or more coreregister(s) within register file 208.

During a core TM tracking interval beginning when the load targetaddress of the transactional load request hits in L1 cache 226,processor core 200 continuously monitors for receipt from the associatedL2 cache 230 of any invalidation message 386 that invalidates the targetcache line from which processor core 200 accessed possibly stale data inresponse to the transactional load (block 734). In addition, processorcore 200 continuously monitors for receipt from L2 cache 230 of a LDAACK message, which signifies that TM tracking logic 381 of L2 cache 230has assumed responsibility for tracking the target cache line address ofthe transactional load (block 738). In response to receipt of aninvalidation message 386 from L2 cache 230 that specifies the targetcache line address for which processor core 200 is monitoring during thecore TM monitoring interval, processor core 200 resets core TM flag 212to indicate that the memory transaction should fail (block 736).

As indicated by the loop from block 738 back to block 734, processorcore 200 continues monitoring for an invalidation message 386 untilprocessor core 200 determines at block 738 that a TM LDA Ack message isreceived from TM tracking logic 381 of L2 cache 230. In response to adetermination at block 738 that a TM LDA Ack message has been received,the core TM tracking interval ends, and the process of FIG. 7 ends atblock 710.

With reference now to FIG. 8, there is illustrated a high level logicalflowchart of an exemplary method of processing transactional loadoperations in a lower level cache in accordance with the presentinvention. The illustrated process begins at block 800 in response to alower level cache memory, such as an L2 cache 230, receiving atransactional load operation (e.g., a TM LD or TM LDA) operation from anaffiliated processor core 200. In response to receipt of thetransactional load operation, L2 cache 230 dispatches an RC machine 312to service the transactional load operation (block 802). In a preferredembodiment, the RC dispatch logic of L2 cache 230 represents a point ofserialization of transactional load and store operations with respect tosnooped operations in that L2 cache 230 defers dispatch of the RCmachine 312 until deallocation of any snoop machine 311 then allocatedto service a snooped operation targeting the same cache line as thetransactional load operation. And in a similar fashion, the presence ofan RC machine 312 servicing a transactional load or operation defersdispatch of a snoop machine 311 to service a snooped operation targetingthe same cache line as the transactional load operation untildeallocation of the RC machine 312. TM tracking logic 381 of L2 cache230 additionally records the target cache line address of thetransactional load operation as part of the transaction footprint of thecurrent memory transaction (block 804).

Once dispatched, RC machine 312 determines at block 806 whether thetransactional load operation is a TM LDA operation. If not (i.e., thetransactional load operation is a TM LD operation that requests returnof the target cache line of data), the process proceeds to block 810,which illustrates the RC machine 312 allocated to service the TM LDoperation obtaining the target cache line of data from the L2 cachearray 302 or, if the target cache line address misses in L2 directory308, system memory 108 or another processing unit 104. Following block810, RC machine 312 returns the target cache line to processor core 200(block 812). Thereafter, L2 cache 230 releases the RC machine 312allocated to service the TM LD operation (block 814), and the process ofFIG. 8 ends at block 816.

Returning to block 806, in response to a determination by RC machine 312that the transactional load operation received from processor core 200is a TM LDA operation, the process proceeds to block 416, whichillustrates RC machine 312 returning a TM LDA Ack message 372 toprocessor core 200. In a preferred embodiment, L2 cache 230 serializesinvalidation messages 386 transmitted to processor core 200 by snoopmachines 311 and LDA ACK messages 372 transmitted by RC machines 312 inorder to ensure that such messages are received by processor core 200 inthe same relative order that RC machines 312 and snoop machines 311 gaincontrol of the target cache line of a transactional load operation.Following block 820, the process of FIG. 8 passes to block 814-816,which have been described.

Referring now to FIG. 9, there is depicted a high level logicalflowchart of the processing of a tend instruction terminating a memorytransaction in accordance with one embodiment. The process begins atblock 900, for example, in response to initiation of execution of a tendinstruction within the LSU 202 of a processor core 200. The process ofFIG. 9 proceeds from block 900 to block 902, which depicts processorcore 200 clearing TM LD Dir 902. Processor core 200 further determinesat block 904 whether or not TM flag 212, which was set to an activestate by the tbegin instruction that initiated the memory transaction,is still in an active state. If a determination is made at block 904that TM flag 212 is not in an active state, meaning that processor core200 received an invalidation message 386 that invalidated the targetcache line address of a TM load operation during the associated core TMmonitoring interval, the process passes to block 912, which is describedbelow. If, on the other hand, processor core 200 determines at block 904that TM flag 212 is still active, meaning that processor core 200 didnot receive an invalidation message 386 that invalidated the targetcache line address of a TM load operation during any core TM monitoringinterval of the current memory transaction, the process proceeds toblock 906.

Block 906 depicts ensuring that the cache line invalidationsnecessitated by transactional stores within the memory transaction havebeen fully propagated throughout the distributed shared memory system.In the embodiment described above, verification of propagation of thecache line invalidations necessitated by transactional stores isaccomplished by one or more snoop machines 311 providing a retryresponse to any applicable TEND request on local interconnect 114 untilthe previous transactional stores have invalidated all cached copies ofthe memory location(s) targeted by the memory updates. The process thenproceeds to step 908, which illustrates ensuring that the cache lineinvalidations necessitated by causally dependent non-transactionalstores have completely propagated throughout the distributed sharedmemory system. In the embodiment described above, verification ofpropagation of the cache line invalidations necessitated bynon-transactional stores is accomplished by one or more instances ofcausality resolution logic 379 providing a retry response to anyapplicable TEND request on local interconnect 114 until the previousmemory updates of causally dependent non-transactional stores haveinvalidated all cached copies of the memory location(s) targeted by thememory updates.

At block 910, transactional control logic 382 of L2 cache 230 determineswhether or not a conflict has occurred for the memory transaction inaccordance with the conflict policy implemented by transactional controllogic 382. In response to transactional control logic 382 determiningthat a conflict has occurred, the process proceeds to block 912, whichdepicts transactional control logic 392 invalidating the tentative storefootprint of the memory transaction (e.g., as recorded in L2 cache 230)and indicating via pass/fail indication 384 that the memory transactionhas failed. As further illustrated at block 912, in response topass/fail indication 384 processor core 200 updates its condition coderegister and transfers control to the fail handling branch instructionwithin the memory transaction. The process then passes to block 916,which is described below.

Returning to block 910, in response to transactional control logic 382determining that no conflict has occurred during execution of the memorytransaction, the process proceeds to step 914, which depicts TM controllogic 382 committing the transaction, inter alia, by causing thetransaction footprint to be committed to the distributed shared memorysystem (e.g., by updating one or more coherence states in the directory308 of L2 cache 230 to indicate the transaction footprint is valid andavailable for access by all threads) and indicating to processor core200 via pass/fail indication 384 that the memory transaction passed.Finally, at block 916, processor core 200 resets TM flag 212 in responseto the pass/fail indication 384, and L2 cache 230 clears the cache lineaddresses from TM tracking logic 381. The process of FIG. 9 thereafterterminates at block 920.

With reference now to FIG. 10, there is depicted a block diagram of anexemplary design flow 1000 used for example, in semiconductor IC logicdesign, simulation, test, layout, and manufacture. Design flow 1000includes processes, machines and/or mechanisms for processing designstructures or devices to generate logically or otherwise functionallyequivalent representations of the design structures and/or devicesdescribed above and shown in FIGS. 1-3 and 6. The design structuresprocessed and/or generated by design flow 1000 may be encoded onmachine-readable transmission or storage media to include data and/orinstructions that when executed or otherwise processed on a dataprocessing system generate a logically, structurally, mechanically, orotherwise functionally equivalent representation of hardware components,circuits, devices, or systems. Machines include, but are not limited to,any machine used in an IC design process, such as designing,manufacturing, or simulating a circuit, component, device, or system.For example, machines may include: lithography machines, machines and/orequipment for generating masks (e.g. e-beam writers), computers orequipment for simulating design structures, any apparatus used in themanufacturing or test process, or any machines for programmingfunctionally equivalent representations of the design structures intoany medium (e.g. a machine for programming a programmable gate array).

Design flow 1000 may vary depending on the type of representation beingdesigned. For example, a design flow 1000 for building an applicationspecific IC (ASIC) may differ from a design flow 1000 for designing astandard component or from a design flow 1000 for instantiating thedesign into a programmable array, for example a programmable gate array(PGA) or a field programmable gate array (FPGA) offered by Altera® Inc.or Xilinx® Inc.

FIG. 10 illustrates multiple such design structures including an inputdesign structure 1020 that is preferably processed by a design process1010. Design structure 1020 may be a logical simulation design structuregenerated and processed by design process 1010 to produce a logicallyequivalent functional representation of a hardware device. Designstructure 1020 may also or alternatively comprise data and/or programinstructions that when processed by design process 1010, generate afunctional representation of the physical structure of a hardwaredevice. Whether representing functional and/or structural designfeatures, design structure 1020 may be generated using electroniccomputer-aided design (ECAD) such as implemented by a coredeveloper/designer. When encoded on a machine-readable datatransmission, gate array, or storage medium, design structure 1020 maybe accessed and processed by one or more hardware and/or softwaremodules within design process 1010 to simulate or otherwise functionallyrepresent an electronic component, circuit, electronic or logic module,apparatus, device, or system such as those shown in FIGS. 1-3 and 6. Assuch, design structure 1020 may comprise files or other data structuresincluding human and/or machine-readable source code, compiledstructures, and computer-executable code structures that when processedby a design or simulation data processing system, functionally simulateor otherwise represent circuits or other levels of hardware logicdesign. Such data structures may include hardware-description language(HDL) design entities or other data structures conforming to and/orcompatible with lower-level HDL design languages such as Verilog andVHDL, and/or higher level design languages such as C or C++.

Design process 1010 preferably employs and incorporates hardware and/orsoftware modules for synthesizing, translating, or otherwise processinga design/simulation functional equivalent of the components, circuits,devices, or logic structures shown in FIGS. 1-3 and 6 to generate anetlist 1080 which may contain design structures such as designstructure 1020. Netlist 1080 may comprise, for example, compiled orotherwise processed data structures representing a list of wires,discrete components, logic gates, control circuits, I/O devices, models,etc. that describes the connections to other elements and circuits in anintegrated circuit design. Netlist 1080 may be synthesized using aniterative process in which netlist 1080 is resynthesized one or moretimes depending on design specifications and parameters for the device.As with other design structure types described herein, netlist 1080 maybe recorded on a machine-readable storage medium or programmed into aprogrammable gate array. The medium may be a non-volatile storage mediumsuch as a magnetic or optical disk drive, a programmable gate array, acompact flash, or other flash memory. Additionally, or in thealternative, the medium may be a system or cache memory, or bufferspace.

Design process 1010 may include hardware and software modules forprocessing a variety of input data structure types including netlist1080. Such data structure types may reside, for example, within libraryelements 1030 and include a set of commonly used elements, circuits, anddevices, including models, layouts, and symbolic representations, for agiven manufacturing technology (e.g., different technology nodes, 32 nm,45 nm, 90 nm, etc.). The data structure types may further include designspecifications 1040, characterization data 1050, verification data 1060,design rules 1070, and test data files 1085 which may include input testpatterns, output test results, and other testing information. Designprocess 1010 may further include, for example, standard mechanicaldesign processes such as stress analysis, thermal analysis, mechanicalevent simulation, process simulation for operations such as casting,molding, and die press forming, etc. One of ordinary skill in the art ofmechanical design can appreciate the extent of possible mechanicaldesign tools and applications used in design process 1010 withoutdeviating from the scope and spirit of the invention. Design process1010 may also include modules for performing standard circuit designprocesses such as timing analysis, verification, design rule checking,place and route operations, etc.

Design process 1010 employs and incorporates logic and physical designtools such as HDL compilers and simulation model build tools to processdesign structure 1020 together with some or all of the depictedsupporting data structures along with any additional mechanical designor data (if applicable), to generate a second design structure 1090.Design structure 1090 resides on a storage medium or programmable gatearray in a data format used for the exchange of data of mechanicaldevices and structures (e.g., information stored in a IGES, DXF,Parasolid XT, JT, DRG, or any other suitable format for storing orrendering such mechanical design structures). Similar to designstructure 1020, design structure 1090 preferably comprises one or morefiles, data structures, or other computer-encoded data or instructionsthat reside on transmission or data storage media and that whenprocessed by an ECAD system generate a logically or otherwisefunctionally equivalent form of one or more of the embodiments of theinvention shown in FIGS. 1-3 and 6. In one embodiment, design structure1090 may comprise a compiled, executable HDL simulation model thatfunctionally simulates the devices shown in FIGS. 1-3 and 6.

Design structure 1090 may also employ a data format used for theexchange of layout data of integrated circuits and/or symbolic dataformat (e.g., information stored in a GDSII (GDS2), GL1, OASIS, mapfiles, or any other suitable format for storing such design datastructures). Design structure 1090 may comprise information such as, forexample, symbolic data, map files, test data files, design contentfiles, manufacturing data, layout parameters, wires, levels of metal,vias, shapes, data for routing through the manufacturing line, and anyother data required by a manufacturer or other designer/developer toproduce a device or structure as described above and shown in FIGS. 1-3and 6. Design structure 1090 may then proceed to a stage 1095 where, forexample, design structure 1090: proceeds to tape-out, is released tomanufacturing, is released to a mask house, is sent to another designhouse, is sent back to the customer, etc.

As has been described, in at least one embodiment, a processing unit fora multiprocessor data processing system includes a processor coreincluding a store-through upper level cache, an instruction executionunit, and a flag. In response to execution in a memory transaction of atransactional load instruction that speculatively binds to a value heldin the store-through upper level cache, the processor core sets theflag, transmits a transactional load operation to a store-in lower levelcache that tracks a target cache line address of a target cache linecontaining the value, monitors, during a core TM tracking interval, thetarget cache line address for invalidation messages from the store-inlower level cache until the store-in lower level cache signals that thestore-in lower level cache has assumed responsibility for tracking thetarget cache line address, and responsive to receipt during the core TMtracking interval of an invalidation message indicating presence of aconflicting snooped operation, resets the flag. At termination of thememory transaction, the processor core fails the memory transactionresponsive to the flag being reset.

While various embodiments have been particularly shown and described, itwill be understood by those skilled in the art that various changes inform and detail may be made therein without departing from the spiritand scope of the appended claims and these alternate implementations allfall within the scope of the appended claims. For example, althoughaspects have been described with respect to a computer system executingprogram code that directs the functions of the present invention, itshould be understood that present invention may alternatively beimplemented as a program product including a computer-readable storagedevice storing program code that can be processed by a processor of adata processing system to cause the data processing system to performthe described functions. The computer-readable storage device caninclude volatile or non-volatile memory, an optical or magnetic disk, orthe like, but excludes propagating signals per se and transmission mediaper se.

As an example, the program product may include data and/or instructionsthat when executed or otherwise processed on a data processing systemgenerate a logically, structurally, or otherwise functionally equivalentrepresentation (including a simulation model) of hardware components,circuits, devices, or systems disclosed herein. Such data and/orinstructions may include hardware-description language (HDL) designentities or other data structures conforming to and/or compatible withlower-level HDL design languages such as Verilog and VHDL, and/or higherlevel design languages such as C or C++. Furthermore, the data and/orinstructions may also employ a data format used for the exchange oflayout data of integrated circuits and/or symbolic data format (e.g.information stored in a GDSII (GDS2), GL1, OASIS, map files, or anyother suitable format for storing such design data structures).

What is claimed is:
 1. A method of data processing in a processing unitincluding a processor core including a store-through upper level cache,an instruction execution unit, and a flag, the method comprising:responsive to the instruction execution unit executing in a memorytransaction a transactional load instruction that speculatively binds toa value held in the store-through upper level cache: setting the flag;transmitting a transactional load operation to a store-in lower levelcache that tracks a target cache line address of a target cache linecontaining the value; monitoring, during a core TM tracking interval,the target cache line address for invalidation messages from thestore-in lower level cache until the store-in lower level cache signalsthat the store-in lower level cache has assumed responsibility fortracking the target cache line address; responsive to receipt during thecore TM tracking interval of an invalidation message indicating presenceof a conflicting snooped operation, resetting the flag; and attermination of the memory transaction, failing the memory transactionresponsive to the flag being reset.
 2. The method of claim 1, whereinthe core TM tracking interval is a variable length interval that beginsin response to said transactional load instruction speculatively bindingto the value held in the store-through upper level cache and ends inresponse to receipt from said store-in lower level cache of anacknowledgement message indicating that the store-in lower level cachehas assumed responsibility for tracking the target cache line address.3. The method of claim 1, and further comprising: responsive to thetransactional load instruction not binding in the store-through upperlevel cache, the processor core refraining from setting the flag andfrom monitoring the target cache line address.
 4. The method of claim 1,and further comprising: responsive to the transactional load operation,tracking the target cache line address utilizing transactional memorylogic in the store-in lower level cache; and responsive to thetransactional memory logic detecting a conflicting access to the targetcache line address, the transactional memory logic transmitting to theprocessor core a fail indication indicating that the memory transactionfailed.
 5. The method of claim 4, wherein: the transactional loadoperation is an address-only transactional load operation that does notrequest return of data; and the transactional memory logic signalingthat the store-in lower level cache has assumed responsibility fortracking the target cache line address includes the transactional memorylogic transmitting an acknowledgement in response to receipt of theaddress-only transactional load operation.
 6. The method of claim 5,wherein: the processor core filtering transmission of address-onlytransactional load operations to the store-in lower level cache byreference to a transactional memory load directory.